The basic principle of lossless audio coding is depicted in FIG. 1. The digital PCM Audio signal samples are not independent to each other. A signal de-correlation 11 is used to reduce this dependency before entropy coding 12. This process needs to be reversible, to be able to restore the original signal. Known de-correlation techniques are using Linear Predictive Filtering (also known as Linear Predictive Coding LPC), integer filter-banks and lossy based approaches.
The basic principle of lossy based lossless coding is depicted in FIG. 2 and FIG. 3. In the encoding part (left side) in FIG. 2, a PCM audio input signal SPCM passes through a lossy encoder 21 to a lossy decoder 22 and as a lossy bit stream to a lossy decoder 25 in the decoding part (right side). Lossy encoding and decoding is used to decorrelate the signal. The output signal of decoder 22 is removed from the input signal SPCM in a subtractor 23, and the resulting difference signal passes through a lossless encoder 24 as an extension bit stream to a lossless decoder 27. The output signals of the decoders 25 and 27 are combined 26 so as to regain the original signal SPCM.
This basic principle is disclosed for audio coding in EP-B-0756386 and U.S. Pat. No. 6,498,811, and is also discussed in P. Craven, M. Gerzon, “Lossless Coding for Audio Discs”, J. Audio Eng. Soc., Vol. 44, No. 9, Sep. 1996, and in J. Koller, Th. Sporer, K. H. Brandenburg, “Robust Coding of High Quality Audio Signals”, AES103rd Convention, Preprint 4621, Aug. 1997.
In the lossy encoder in FIG. 3, the PCM audio input signal SPCM passes through an analysis filter bank 31 and a quantisation 32 of sub-band samples to a coding and bit stream packing 33. The quantisation is controlled by a perceptual model calculator 34 that receives signal SPCM and corresponding information from the analysis filter bank 31. At decoder side, the encoded lossy bit stream enters a means 35 for de-packing the bit stream, followed by means 36 for decoding the subband samples and by a synthesis filter bank 37 that outputs the decoded lossy PCM signal SDec. Examples for lossy encoding and decoding are described in detail in the standard ISO/IEC 11172-3 (MPEG-1 Audio).
Because a lossy encoder produces an error signal SDiff that is proportional to the masking thresholds in the frequency domain, the signal is not very well de-correlated and therefore sub-optimum for entropy coding. As a consequence, the following publications focus on a special handling of the error signal SDiff. The common approach is to apply variations of LPC de-correlation schemes to the error signal SDiff: WO-A-9953677, U.S. Pat. No. 20040044520, WO-A-2005098823. In EP-A-0905918 the amplitude of the error signal SDiff is used with a feedback loop to the quantisation stage of the lossy encoder part in order to control the quantisation in the lossy encoder and thus to generate a better de-correlation of the error signal SDiff.